Method for making magnetic particles having improved conductivity and their use in electrostatographic printing applications

Abstract

The present invention provides for a method for the preparation of red colored magnetic particles for multicomponent toner compositions which are highly conductive and therefore suitable for use as developers in high speed electrophotographic copy machines embodying magnetic brush development. Suitable magnetic core metal particles are provided which are subsequently coated with finely divided particles of copper oxide (CuO). The copper oxide coating is then reduced in-situ on the surface of the core particle to provide an electrically conductive core particle uniformly coated with adherent metallic copper, which particle is of a red color. The present invention also provides a method for controlling and adjusting the electrical conductivity and color of toner materials as a function of the amount of metallic copper deposited on the surface of the magnetic core materials.

What is claimed is: 
     
       1. A process for making conductive colored magnetic particles comprising: a) providing magnetic core particles comprising a finely divided metal oxide, said particles having an average particle size within the range of from about 1 to about 50 microns,   b) depositing finely divided submicron size particles of copper oxide on the surface of said core particles, and   c) reducing said copper oxide to metallic copper.   
     
     
       2. The process of claim 1 wherein said deposition of step (b) is carried out by mixing said magnetic core particles with a solution of a water or alcohol soluble salt of copper and precipitating particles of copper in the form of copper oxide on the surface of said core particles. 
     
     
       3. The process of claim 2 wherein said precipitation is carried out by evaporating said solution of water soluble salt and heating the residue at a temperature sufficient to decompose said water soluble salt to copper oxide. 
     
     
       4. The process of claim 1 wherein said magnetic core particles are composed of gamma Fe 2  O 3 . 
     
     
       5. The process of claim 3 wherein said heating is conducted at a temperature within the range of from about 200° to about 400° C. 
     
     
       6. The process of claim 3 wherein said copper oxide is reduced by heating the core particles coated with copper oxide in a stream of hydrogen gas and at a temperature below about 200° C. for a period of time sufficient to reduce said copper oxide to metallic copper. 
     
     
       7. The process of claim 1 wherein the amount of copper oxide present on the surface of said core particles constitutes from about 5 to about 60% by weight based on the total weight of the coated core particles. 
     
     
       8. Finely divided conductive colored magnetic particles produced by the process of claim 1. 
     
     
       9. The particles of claim 8 having a resistivity within the range of from about 1 to about 100 ohms. 
     
     
       10. A colored particulate toner composition comprising a uniform mixture of a fusible binder resin having the colored magnetic particles of claim 8 dispersed therein.